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Duck-Billed Platypus Genome Sequence Published

The first analysis of the genome sequence of the duck-billed platypus, whose ancestors split from the rest of mammalian lineage some 166 million years ago, revealed clues about how genomes were organized during the early evolution of mammals.

Image courtesy of NHGRI.

The duck-billed platypus, native to Australia, is one of the
few mammals that lays eggs. It has other peculiarities, too—for
example, it boasts what looks like a duck's bill and houses an
electrosensory system used when foraging for food underwater.
Males also have venomous hind leg spurs.

"At first glance, the platypus appears as if it was the
result of an evolutionary accident," said Dr. Francis S.
Collins, director of NIH's National Human Genome Research Institute
(NHGRI). "But as weird as this animal looks, its genome
sequence is priceless for understanding how fundamental mammalian
biological processes have evolved."

In a paper published in the May 8, 2008, issue of the journal Nature, an
international team of researchers supported in part by NHGRI
analyzed a high-quality draft genome sequence of Glennie, a female
platypus from Australia. The sequencing was led by the Genome
Sequencing Center at Washington University School of Medicine
in St. Louis, part of NHGRI's Large-Scale Sequencing Research
Network. The researchers then compared the genome with the well-characterized
genomes of the human, mouse, dog, opossum and chicken, as well
as the draft genome sequence of the green anole lizard.

The team found that the platypus genome contains about the
same number of protein-coding genes as other mammals—approximately
18,500. The platypus shares more than 80% of its genes with other
mammals whose genomes have been sequenced.

Like other mammals, the platypus genome contains a tightly
clustered set of genes that produce the casein proteins that
make up milk. However, the platypus genome harbors both reptilian
and mammalian genes associated with the fertilization of eggs—particularly
interestingly, since the female platypus lays eggs.

The researchers found several other unique characteristics
to this odd mammal's genome. For example, the platypus has about
half as many odor receptor genes as mice and other mammals, but
more genes that code for a particular type of odor receptor called
a vomeronasal receptor. Researchers think that this particular
set of genes may be involved in detecting odors during underwater
foraging.

The genes related to the immune system of the platypus are
very similar to those of other mammals. However, the platypus
genome has a few key differences that the researchers believe
may help boost the immune systems of their offspring, which hatch
from the egg at a less developmentally mature stage than other
mammals when they are born.

The platypus genome sequence is now freely available online for
other researchers to use. “This genome provides a unique perspective
on what the genomes of our earliest mammalian ancestors may have
looked like,” said Dr. Adam Felsenfeld, who heads NHGRI's
Comparative and Sequencing Analysis Program. “It is fascinating
that what we think of as being reptile-like and mammal-like features
can co-exist in the same genome.”